Standard Tetrahedral Bond Radii

In 1926 M. H. Huggins published a set of bonding radii for twelve main-group elements having four electron pairs in the valence shell based on seventeen crystal stmctures including those of the four Group 14 elements referred to above [55]. 

Zinc sulfide is found in two crystalline forms, sphalerite (zinc blende) and wurtzite. The stmcture of the former is analogous to that of SiC. The second, wurtzite, is also constracted from perfectly tetrahedral ZnS4 (or SZn4) units, but 

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The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

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